A&A 596, A67 (2016) Astronomy DOI: 10.1051/0004-6361/201527947 & c ESO 2016 Astrophysics SN 2012aa: A transient between Type Ibc core-collapse and superluminous supernovae R. Roy1, J. Sollerman1, J. M. Silverman2, A. Pastorello3, C. Fransson1, A. Drake4, F. Taddia1, C. Fremling1, E. Kankare5, B. Kumar6, E. Cappellaro3, S. Bose7, S. Benetti3, A. V. Filippenko8, S. Valenti9, A. Nyholm1, M. Ergon1, F. Sutaria6, B. Kumar7, S. B. Pandey7, M. Nicholl10, D. Garcia-Alvarez´ 11; 12; 13, L. Tomasella3, E. Karamehmetoglu1, and K. Migotto1 1 The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden e-mail: [email protected]; [email protected] 2 Department of Astronomy, University of Texas, Austin, TX 78712-0259, USA 3 INAF−Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy 4 California Institute of Technology, 1200 E. California Blvd., CA 91225, USA 5 Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, BT7 1NN, UK 6 Indian Institute of Astrophysics, Koramangala, 560 034 Bangalore, India 7 Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, 263 129 Nainital, India 8 Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA 9 Department of Physics, University of California, Davis, CA 95616-8677, USA 10 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 11 Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain 12 Dpto. de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain 13 Grantecan CALP, 38712, Brena¯ Baja, La Palma, Spain Received 11 December 2015 / Accepted 1 July 2016 ABSTRACT Context. Research on supernovae (SNe) over the past decade has confirmed that there is a distinct class of events which are much more luminous (by ∼2 mag) than canonical core-collapse SNe (CCSNe). These events with visual peak magnitudes .–21 are called superluminous SNe (SLSNe). The mechanism that powers the light curves of SLSNe is still not well understood. The proposed scenarios are circumstellar interaction, the emergence of a magnetar after core collapse, or disruption of a massive star through pair production. Aims. There are a few intermediate events which have luminosities between these two classes. They are important for constraining the nature of the progenitors of these two different populations and their environments and powering mechanisms. Here we study one such object, SN 2012aa. Methods. We observed and analysed the evolution of the luminous Type Ic SN 2012aa. The event was discovered by the Lick Observatory Supernova Search in an anonymous galaxy (z ≈ 0:08). The optical photometric and spectroscopic follow-up observations were conducted over a time span of about 120 days. Results. With an absolute V-band peak of ∼−20 mag, the SN is an intermediate-luminosity transient between regular SNe Ibc and SLSNe. SN 2012aa also exhibits an unusual secondary bump after the maximum in its light curve. For SN 2012aa, we interpret this as a manifestation of SN-shock interaction with the circumstellar medium (CSM). If we assume a 56Ni-powered ejecta, the quasi- 56 bolometric light curve requires roughly 1.3 M of Ni and an ejected mass of ∼14 M . This also implies a high kinetic energy of the explosion, ∼5:4 × 1051 erg. On the other hand, the unusually broad light curve along with the secondary peak indicate the possibility of interaction with CSM. The third alternative is the presence of a central engine releasing spin energy that eventually powers the light curve over a long time. The host of SN 2012aa is a star-forming Sa/Sb/Sbc galaxy. Conclusions. Although the spectral properties of SN 2012aa and its velocity evolution are comparable to those of normal SNe Ibc, its broad light curve along with a large peak luminosity distinguish it from canonical CCSNe, suggesting that the event is an intermediate- luminosity transient between CCSNe and SLSNe at least in terms of peak luminosity. In comparison to other SNe, we argue that SN 2012aa belongs to a subclass where CSM interaction plays a significant role in powering the SN, at least during the initial stages of evolution. Key words. supernovae: general – supernovae: individual: SN 2012aa 1. Introduction Panoramic Survey Telescope & Rapid Response System (Pan- STARRS; Hodapp et al. 2004). SLSNe are much more lumi- The study of superluminous supernovae (SLSNe; Gal-Yam nous than normal core-collapse SNe (CCSNe; Filippenko 1997). 2012) has emerged from the development of untargeted transient It was with the discovery of objects such as SNe 2005ap and surveys such as the Texas Supernova Search (Quimby 2006), the SCP-06F6 (Quimby et al. 2007, 2011) as well as SN 2007bi Palomar Transient Factory (PTF; Rau et al. 2009), the Catalina (Gal-Yam et al. 2009) that events with peak luminosities Real-time Transient Survey (CRTS; Drake et al. 2009), and the Article published by EDP Sciences A67, page 1 of 21 A&A 596, A67 (2016) &7 × 1043 erg s−1 (.−21 absolute mag, over 2 mag brighter than appearance of a short-lived bump before the principal light-curve the bulk CCSN population) became known. peak (Anupama et al. 2005). Two hypotheses have been pro- SLSNe have been classified into three groups: SLSN-I, posed to explain this early short-duration bump: either a burst SLSN-II, and SLSN-R (Gal-Yam 2012). The SLSN-II are hydro- of radiation from a blob containing radioactive 56Ni that was gen (H)-rich (e.g., SN 2006gy, Smith et al. 2007; CSS100217, ejected asymmetrically (Maeda et al. 2007) or a polar explosion Drake et al. 2011; CSS121015, Benetti et al. 2014), while the powered by a relativistic jet (Folatelli et al. 2006). Although a others are H-poor. The SLSN-R (e.g., SN 2007bi, Gal-Yam double-peaked light curve was only observed in a single SN Ic, et al. 2009) have post-maximum decline rates consistent with there are at least a handful (∼8) of Type I SLSNe that exhibit the 56Co ! 56Fe radioactive decay, whereas SLSNe-I (e.g., a short-duration bump before the principal broad peak (Nicholl SNe 2010gx, Pastorello et al. 2010b; SCP-06F6) have steeper de- & Smartt 2016). Most of them have been described using the clines. Hydrogen-poor events mostly exhibit SN Ic-like spectral magnetar model. For SN-LSQ14bdq it was proposed that the in- evolution (Pastorello et al. 2010b; Quimby et al. 2011; Inserra tense radiation in optical and ultraviolet wavelengths was due et al. 2013). However, the explosion and emission-powering to the emergence of the shock driven by a high-pressure bubble mechanisms of these transients are still disputed. It is also not produced by the central engine, a proto-magnetar (Kasen et al. clear whether CCSNe and SLSNe originated from similar or 2016). completely different progenitor channels, or if there is a link Since H-poor SLSNe and normal Type Ic events show sim- between these kinds of explosions. In particular, the SLSNe-R ilar spectral features beyond maximum light, it is worth ex- were initially thought to be powered by radioactive decay, but ploring whether there is any link between them. The gap be- later the energy release of a spin-down magnetar was proposed (Inserra et al. 2013), as was the CSM-interaction scenario (Yan tween SNe and SLSNe has not been well explored. The Type Ib et al. 2015; Sorokina et al. 2016). In this work, although we use SN 2012au shows spectral signatures which mark the transition the notation of the initial classification scheme, by SLSNe-R we between H-poor CCSNe and SLSNe, although its peak mag- simply mean those Type Ic SLSNe which show shallow (or com- nitude was comparable to those of H-poor (stripped envelope) parable to 56Co decay) decline after maximum brightness, while CCSNe (Milisavljevic et al. 2013). The luminosity of broad line the events which decline faster than 56Co decay are designated Type Ic SN 2011kl, associated with GRB 111209A also fall in between CCSNe and SLSNe (Greiner et al. 2015; Kann et al. SLSNe-I. 2016). Recently, a few transients such as PTF09ge, PTF09axc, The basic mechanisms governing the emission of stripped- PTF09djl, PTF10iam, PTF10nuj, and PTF11glr with peak ab- envelope CCSNe are relatively well known. At early times ( 5 d . solute magnitudes between −19 and −21 have been discovered after explosion), SNe Ibc are powered by a cooling shock Piro & (Arcavi et al. 2014). Except PTF09ge, all had H-dominated spec- Nakar(2013), Taddia et al.(2015). Thereafter, radioactive heat- tra, and except PTF10iam they showed spectral behaviour like ing (56Ni ! 56Co) powers the emission from the ejecta. Beyond that of tidal disruption events (TDEs; Rees 1988). However, the peak luminosity, the photosphere cools and eventually the we note that TDEs may be even more luminous than SLSNe; ejecta become optically thin. By ∼100 d post explosion the lu- for example, the ROTSE collaboration found an event (nick- minosity starts to decrease linearly (in mag) with time, and it is named Dougie) that had a peak magnitude of ∼−22:5 (Vinkó believed that 56Co is the main source powering the light curve et al. 2015). Unlike CCSNe or SLSNe, TDEs are always found during these epochs (Arnett 1982, and references therein). at the center of their host galaxies, and their spectral features Three different models have been suggested for the explo- are normally dominated by H/He emission lines, thus differing sion mechanism of SLSNe. A pair-instability supernova (PISN) 56 from Type Ic SNe. Arcavi et al.(2016) also found four SNe hav- produces a large mass of radioactive Ni, a source of enor- ing rapid rise times (∼10 d) and peak magnitudes of ∼−20 along mous optical luminosity (Barkat et al.